1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention relates to an image forming apparatus of the type including an image carrier and an intermediate image transfer body facing each other to form a primary image transfer region, effecting primary transfer of a toner image from the image carrier to the intermediate image transfer body, and causing the intermediate image transfer body to again convey the image via the primary image transfer region before secondary transfer of the image to a recording medium.
2. Discussion of the Background
It has been customary with an image forming apparatus of the type described to vary the moving speed of the surface of an intermediate image transfer body at the time of secondary image transfer, depending on whether a recording medium to be used is a plain sheet or a thick or OHP (OverHead Projector) sheet. When use is made of, e.g., a thick sheet, the surface of the intermediate image transfer body is moved at a speed approximately one half of a speed assigned to a plain sheet. This is because an electric field formed in a secondary image transfer region where the intermediate image transfer body and a recording medium face each other becomes weaker when the medium is a thick sheet than when it is a plain sheet, resulting in short image transfer.
The above image forming apparatus includes a controller for controlling the moving speed of the surface of the intermediate image transfer body. Further, the controller accurately determines the position of a toner image completed on the intermediate image transfer body by the primary transfer by referencing the output of angular position sensing means. More specifically, a mark sensor playing the role of the angular position sensing means senses a mark provided on the intermediate image transfer body and sends its output to the controller. The controller by so determine the position of the image controls, e.g., the movement of a cleaning member into and out of contact with the intermediate image transfer body and the operation timing of a registration roller pair used to convey the recording medium to a secondary image transfer region.
As for the operation timing of the registration roller, for example, the controller calculates an interval between the time when it recognizes the position of the image and the time when the leading edge of the image arrives at the secondary image transfer region on the basis of the moving speed of the surface of the intermediate image transfer body. The controller drives the registration roller pair in accordance with the calculated interval. More specifically, the controller recognizes the position of the image on the basis of a period of time elapsed since the intermediate image transfer body has moved away from a sensing position where the mark sensor and the inner surface of the intermediate transfer body face each other.
The problem with the apparatus of the type executing sequence control on the basis of the above period of time is that when the moving speed of the surface of the intermediate image transfer body is switched, as stated above, a difference occurs between the position of the image recognized by the controller and the actual position of the image. Therefore, to effect secondary image transfer with a thick sheet or similar special sheet, the intermediate image transfer body carrying the image transferred thereto by the primary transfer must again bring its mark to the sensing position, so that the controller can again recognize the position of the image. This has customarily been done by causing the intermediate transfer body to run idle such that the image formed thereon is returned to the primary image transfer region via the secondary image transfer region before the secondary image transfer.
When the intermediate image transfer body again conveys its image to the primary image transfer region while running idle, charge depositing means included in the conventional apparatus for forming an electric field for the primary image transfer applies a bias of the same size as a bias for forming the above electric field. This undesirably increases a potential gap in the primary image transfer region, i.e., a difference between the bias to be deposited by the charge depositing means and the surface potential of the image carrier. When toner carried on the intermediate image transfer body enters the above region where the potential gap is great, and particularly when the body has a medium volume resistivity, it is likely that a charge is injected into the toner and charges it to a positive polarity. The toner so reversed in polarity is partly returned from the intermediate image transfer body to the image carrier in the primary image transfer region, resulting in a low density, vermicular image. This is particularly true with recycled toner whose amount of charge has been reduced.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-186860 and 7-225520.